Structural and functional properties will be examined for several proteins of physiological interest: hemoglobin, cytochromes, calcium binding protein, and polypeptide hormones such as human chorionic somatomammotropin and its pituitary analogs growth hormone and prolactin. Biophysical and biochemical techniques to be employed include macromolecular X-ray crystallography, stopped-flow flash photolysis kinetics, fluorescence, and both EXAFS and diffraction techniques which exploit the unique properties of synchrotron radiation. For each protein, we are interested in those aspects of its structure which are particularly related to its physiological function. Thus for hemoglobin and cytochromes, we concentrate on the heme group and its interaction with the surrounding protein; for calcium binding protein, on its interaction with calcium and other ions; and for the hormones, on their overall structure. Hemoglobin is involved in respiration, the transport of oxygen and carbon dioxide between the lungs and the tissues; cytochromes, in electron transport and cellular energy production; calcium binding protein, in intestinal absorption and translocation of calcium; and hormones such as human chorionic somatomammotropin, growth hormone and prolactin, in growth-promoting and lactogenic activities.